1. Field of the Invention
This invention relates to a method of limiting a band of a moving-picture signal. This invention also relates to an apparatus for limiting a band of a moving-picture signal.
2. Description of the Prior Art
Motion-compensated predictive encoding is one of highly-efficient encoding of a digital moving-picture signal. According to motion-compensated predictive encoding, every frame represented by a moving-picture signal is divided into blocks of a same size, and signal processing is executed block by block. Specifically, motion-compensated prediction is implemented by using a reference frame represented by picture data which results from decoding a previously-encoded frame. Calculation is given of a predictive error between a current block and a predicted block which results from the motion-compensated prediction. The predictive error is encoded. Thus, motion-compensated predictive encoding compresses a moving-picture signal by using a temporal correlation between successive frames represented by the moving-picture signal.
According to a typical type of the encoding of a predictive error, the predictive error is subjected to orthogonal transform, and a signal which results from the orthogonal transform is quantized. Further, a quantization-resultant signal is subjected to an entropy encoding process. Thus, the typical type of the encoding of a predictive error compresses picture information by using a spatial correlation and a statistical correlation in a moving picture.
In general, a temporal correlation, a spatial correlation, and a statistical correlation considerably vary from picture to picture. On the other hand, an amount of data (the number of bits of data) generated by encoding per unit time, that is, an encoding-resultant-data rate, is generally required to be a constant value independent of the contents of a moving picture. To meet such a requirement, the characteristics of quantization are changed in response to the characteristics of a moving picture.
Specifically, in the case of pictures related to high temporal, spatial, and statistical correlations, fine quantization is executed to increase an encoding-resultant-data rate (the number of bits of data generated by encoding per unit time) to a desired rate. In the case of pictures related to low temporal, spatial, and statistical correlations, coarse quantization is executed to decrease an encoding-resultant-data rate (the number of bits of data generated by encoding per unit time) to the desired rate.
In general, an encoding side and a decoding side are connected via a transmission line. The decoding side receives the output signal of the encoding side via the transmission line, and recovers an original moving-picture signal by decoding the output signal of the encoding side. Quantization at the encoding side causes a quantization distortion in every picture represented by the moving-picture signal recovered at the decoding side. The quantization distortion appears in the form of noise referred to as mosquito noise or block noise.
According to a prior-art method designed to solve such a noise problem, temporal and spatial correlations in moving pictures are measured on the basis of an inter-frame difference (an inter-frame error) at a stage preceding a compressively encoding section, and signal bands in a temporal direction (a direction along a time base) and a spatial direction are limited in response to information of the measured correlations. The prior-art method disregards a temporal redundancy in moving pictures which is generally removed by motion-compensated prediction. Thus, in given signal conditions, the prior-art method needlessly limits the signal bands.